What Are Self Tapping Screws / How Do I Use Self Tapping Screws?
Arguably, the image of a self tapping fastener is one of the most archetypal images of a screw. Many will envision the definitive ‘screw’ as a countersunk screw, usually with a Phillips drive and a sharpened point. A quick Google search for the term ‘screws’ shows an overwhelming bias towards self tapping fasteners, despite the innumerable permutations available. Despite its apparent popularity, ‘How do I use a self-tapping screw?’ is still a very common question - some of the basics will be covered within this article.
What is The Difference Between Self Tapping and Self Drilling Screws?
Tapping is the act of creating a thread. Tapping is not the act of creating a hole. In soft materials, such as wood or plastic, there is some overlap in these processes, as a self tapping tip is usually sufficient to create the hole required. However, with metals or certain particularly hard woods, a pilot hole must be drilled before a thread can be tapped.
Selecting The Correct Drive Type
An important consideration in any self tapping application is to select the ideal drive type for the given situation and the tooling available. Common choices include slotted or Phillips drives - or the Pozi drive, which was specifically invented for self tapping applications. For more information see our article on the advantages of the Pozi drive. Deeper socket drives are also regularly used in high-torque applications.
When installing a self tapping screw, the screw must be properly aligned to ensure that it is driven into the target housing as the correct angle. This is particularly important if a self tapping screw is expected to cut its own hole, as there is no pilot hole to guide the fastener. Many methods exist for ensuring perpendicular holes, from improvised jigs to high-precision guidance tooling.
Many factors go into the selection of an ideal pilot hole, including some degree of preference in the amount of thread engagement required. Factors such as fastener material, housing material, hole depth, and even temperature must be considered. For these reasons, it is near impossible to guarantee a recommended pilot hole size for your specific application. As a rough guide, a pilot hole should be between the minor diameter and major diameter of the screw thread, but will always vary dependent on external factors. The table below gives several recommended hole sizes, but should be used as a guide only.
|Self Tapping Thread Gauge||Sheet Thickness||Pilot Hole Size*|
|No. 4 (2.9mm)||0.45mm||2.21mm|
|No. 6 (3.5mm)||0.45mm||2.70mm|
|No. 8 (4.2mm)||0.71mm||3.00mm|
|No. 10 (4.8mm)||0.71mm||3.51mm|
*Figures given for stainless steel fasteners used in unhardened metals such as mild steel, brass, aluminium etc. Use as a guide only.
How to Use Self Tapping Screws with Wood
As mentioned above, when used with sufficiently soft materials, a self tapping screw can be used to create and tap its own hole. However, a pilot hole is often recommended, particularly in hardwoods, to prevent splitting, which is caused by stresses within the wood. Wood screws are designed with very coarse threads to reduce stresses within wooden housings.
When using self tapping screws with wood, the fastener material choice should also be considered. A corrosion resistant fastener such as stainless steel is advised, as moisture can be absorbed and excreted by wood over time, potentially causing rust on unprotected screws.
How to Use Self Tapping Screws with Plastic
Certain types of self-tapping screw have been specially designed for use with plastics. While regular self tapping screws are suitable for use with most plastics, Plastite screws feature more pronounced, sharper threads, which increase resistance to pull-out and stripping within low-density plastics.
Plastic housings are commonly used alongside thread forming screws, a type of self-tapping screw which does not cut its own threads with a sharp edge, but pushes threads into a housing through deformation of material. For this reason, thread forming screws are only generally only a viable option with more elastic materials such as plastics.
The advantage of thread forming self-tapping screws is that by displacing material, they create a zero tolerance fit which is more resistant to loosening. The downside of thread forming screws is that they require greater torque to install, and can cause disruptive internal stresses.
How to Use Self Tapping Screws with Metal
When used with metal, Self-tapping screws should always be installed into pre-drilled holes. Stainless steel screws are relatively soft when compared to hardened steel, and are generally recommended for use with softer metals, such as Aluminium.
It is important to exercise caution when installing self-tapping screws into metals due to the increased density and hardness of material. When used in metal, self tapping screws have an increased potential to become damaged.
Thread rolling screws also exist for use in metal, though they are generally only used alongside softer metals. Thread rolling screws in metal rely on cold flow of material after the fastener has been installed to create an excellent resistance to loosening. Thread rolling screws in metal are affected by all of the same drawbacks as their plastic counterparts mentioned above - greater requirements for installation torque, and the creation of potentially damaging internal stresses.